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trH3N2 Detection and Reporting Delays Raise Concerns
Recombinomics Commentary 20:30
August 6, 2011

The two cases from Minnesota occurred in a father (index case) and child. The fatherhad a nasopharyngeal swab positive for swine-origin influenza A (H3N2) virus and had direct swine exposure 6 days before illness onset. The child, whose infection with influenza A (H3N2) virus was confirmed several weeks later by serologic testing, did not have direct swine exposure, and most likely acquired infection from close contact with her father. Other persons in the same household also had ILI during the same period, but serologic results were either negative or inconclusive.
The above description of the Minnesota trH3N2 cluster highlights detection and reporting problems associated with trH3N2 cases.  Yesterday’s week 30 MMWR reported a second trH3N2 cases in 2011, and it is likely that the reported cases was the daughter described above (who was infected in 2010 but not reported until in 2011.  The above report was from week 21 MMWR and the case was confirmed by serological testing.  It is likely that virus was isolated recently, leading to the report of the second cases in 2011.  This confirmation may have also led to the use of the isolate from the index case as the pandemic H3N2 (trH3N2) vaccine target.

Like the first cases reported in 2011, the index case’s daughter was not hospitalized and the lab confirmation was difficult.  The other 2011 case was from a September 6, 2010 infection that was reported in week 4 of 2011.  Two of the three late 2010 infections that were reported in 2010 were from hospitalized cases, which were more likely to be tested beyond a rapid test or a serotype test, both of which would suggest the trH3N2 cases were seasonal H3N2, because trH3N2 has a human H3 and N2 from the 1990’s.  Thus, these samples could be detected in a H3 serological test, although the signal may be low, and many cases would be unsubtypable, as reported by Pennsylvania where two of the confirmed cases were located (A/Pennsylvania/40/2010 and A/Pennsylvania/14/2010).

The delayed reporting for the above two 2010 cases is similar to reporting for the 2009 Iowa case (A/Iowa/16/2009), which was from a September infection that was analyzed in November and reported in January, 2010.  These delayed cases not only were reported in the year after infection, but infections were early in the season, prior to the large influx of seasonal cases which would have decreased the likelihood of the more extensive testing, which is required for the detection of trH3N2.

The genetic similarities seen in the human cases suggests the number of cases is much higher than the small number of reported cases and it is likely that the number of cases being investigated is significantly higher than the two cases cited in the 2011 week 30 MMWR, which are likely to be cases infected in 2010.

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